A variety of photographic imaging systems are known wherein a hardenable organic component containing ethylenic unsaturation sites is relied upon for image formation. The organic component undergoes photoinduced addition reactions, typically either polymerization or crosslinking, at the ethylenic unsaturation sites which produce hardening and allow image discrimination to be achieved.
A simple illustration of such an imaging system is a negative working photoresist which contains an imaging dye. Imagewise exposure of the photoresist followed by development leaves a dye image in exposed areas. While there are a plethora of known negative working photoresists which might be cited, Tan et al U.S. Pat. No. 4,289,842 illustrates photoresist employing crosslinkable copolymers containing ethylenic unsaturation in pendant groups, Lindley U.S. Pat. No. 4,590,147 illustrates photoresists including vinyl oligomers, and Fuerniss U.S. Pat. NO. 4,497,889 illustrates photoresists containing vinyl monomers.
Illustrative of more elaborate imaging systems capable of producing transferred dye images are Sanders et al. U.S. Pat. Nos. 4,399,209 and 4,440,846. A dye precursor and a hardenable organic component containing ethylenic unsaturation sites are coated together on a support in rupturable microcapsules. Imagewise exposure to actinic radiation hardens the organic component by inducing addition at its ethylenic unsaturation sites. Upon subsequent rupture of the microcapsules only the dye precursor in unexposed microcapsules have the mobility to transfer to a receiving sheet where a viewable dye image is formed.
Since the hardenable organic components containing ethylenic unsaturation exhibit only limited direct response to exposing radiation, it is common practice to include an initiator of the ethylenic addition reaction. In practice negative working photoresists are typically imagewise exposed using radiation wavelengths in the near ultraviolet region of the spectrum. While Sanders et al recognize that imaging exposures are possible with various forms and wavelengthss of radiation, the preferred wavelengths of exposure are limited to the ultraviolet and the blue, up to about 480 nm, with the initiators for the ethylenic addition reaction disclosed being those which are responsive at these wavelengths of exposure.
In order to achieve higher levels of sensitivity (i.e., higher imaging speeds) than can be achieved with a single initiator, it is common practice in preparing imaging compositions to employ coinitiators. One of the coinitiators is a photosensitizer. Photosensitizers are relied upon to capture photons of exposing radiation. The remaining coinitiator is referred to as an activator. The activator is not relied upon to respond directly to exposing radiation, but rather adjacent activator and photosensitizer molecules react, following excitation of the latter by photon capture, causing release of a free radical which in turn induces immobilizing addition reactions at sites of ethylenic unsaturation.
Imaging systems which rely on a combination of an activator and a photosensitizer are typically exposed in the ultraviolet or blue portion of the spectrum. The necessity of using shorter imaging wavelengths places constraints on the master or pattern employed for imaging. For example, a master which presents a longer wavelength visible dye image, but exhibits little variance in ultraviolet or blue transmittance is not well suited for imagewise exposing an imaging system which responds only to ultraviolet or blue radiation. Further, such imaging systems are disadvantageous and have found limited acceptance in producing multicolor images.
Activators are recognized in the art to fall into two distinct classes. One class of activator is referred to as electron acceptor activators. These activators liberate a free radical capable of initiating ethylenic addition by accepting an electron from a photosensitizer in its excited state. The reactions can be diagrammed as follows: ##STR1## where
Sens represents a photosensitizer,
.lambda. represents exposure to actinic radiation,
* indicates the excited state produced by light absorption on exposure,
A.sup.+ --R represents the electron acceptor activator,
A.sup.+ represents the electron accepting moiety of the activator before an electron is accepted,
A represents the electron accepting moiety of the activator after an electron is accepted, R represents the moiety that is ultimately cleaved as a free radical,
Sens.sup.+.multidot. shows the photosensitizer converted to a cation radical by loss of an electron, and
.multidot. denotes a radical.
The reaction sequence described above is an ideal one in which the electron acceptor activator radical produced by step (b) reacts further as indicated in step (c) to provide a free radical. ignored in this reaction sequence is a possible recombination of the photosensitizer cation radical with the electron acceptor activator radical to regenerate the electron acceptor activator in its initial form and to transform the photosensitizer cation radical into an unexcited dye molecule. This reaction sequence can be diagrammed as follows: ##STR2## Photon energy is internally dissipated in this reaction sequence, since no free radical is produced and the photosensitizer produced is no longer in an excited state. More efficient imaging systems are those in which reaction step (c) is favored over reaction step (d).
Another class of activator is referred to as electron donor activators. These activators liberate a free radical capable of initiating ethylenic addition by donating an electron to a photosensitizer in its excited state. The reactions can be diagrammed as follows: ##STR3## where
D.sup.- --R' represents the electron donor activator,
D.sup.- represents the electron donating moiety of the activator before donating an electron,
D represents the electron donating moiety of the activator after donating an electron,
Sens.sup.- shows the photosensitizer converted to a anion by loss of an electron, and
the remaining symbols are as indicated above.
Again, reaction sequence (III) is an ideal one in which the electron donor radical produced by step (f) reacts further as indicated in step (g) to provide a free radical. Ignored in this reaction sequence is a possible reaction of the photosensitizer anion radical with the electron donor activator radical to regenerate the electron donor activator in its initial form and to transform the photosensitizer cation radical into an unexcited dye molecule. This reaction sequence can be diagrammed as follows: ##STR4## Photon energy is internally dissipated in this reaction sequence, since no free radical is produced and the regenerated photosensitizer is no longer in an excited state. More efficient imaging systems are those in which reaction step (g) is favored over reaction step (h).
A survey of useful electron acceptor and electron donor activators useful with photosensitizers is set forth in Volman et al, Advances in Photochemistry, Vol. 13, in the chapter titled "Dye Sensitized Photopolymerization" by D. F. Eaton, pp. 427 to 488, John Wiley & Sons (1986).
Specht and Farid published U.K. Specification No. 3,083,832A discloses photopolymerization coinitiators including an azinium electron acceptor activator and, acting as a photosensitizer, an amino-substituted ketocoumarin.
Gottschalk et al published European Patent Applications No. 0,223,587 discloses a polymerization system including as an initiator a zwitterion consisting of an undissociated borate anion and a dye cation. Disclosed cationic dyes include Methylene Blue, Safranine O, Malachite Green, and various cyanine and rhodamine dyes.